SiC has a wide band gap when compared to Si (silicon), and has high insulated breakdown field strength. Therefore, SiC is expected to serve as material of high-voltage semiconductor devices. Since the lattice constant of 3C-SiC (SiC which has a 3C type crystal structure) is similar to the lattice constant of GaN (gallium nitride), 3C-SiC can be used as a buffer layer for growing GaN. When 3C-SiC is used as a buffer layer for growing GaN, higher-voltage semiconductor devices of GaN can be obtained, since GaN and 3C-SiC have high insulated breakdown field strength.
As foundation substrates for growing SiC layers, Si substrates or bulk SiC substrates are widely used. Since there are only about 4 inch SiC substrates at present, it is difficult to increase the diameter. To obtain an inexpensive and large diameter SiC layer, it is preferable that an Si substrate is used as the foundation substrate.
The below Document 1 discloses a method for manufacturing a semiconductor substrate having a first step and a second step, wherein a film of AlxInyGa1-x-yN layer (0<x≤1, 0≤y≤1, x+y≤1) is formed on a film of a SiC layer formed on a Si substrate at a temperature higher than a GaN film forming temperature, and a film of GaN is formed at the GaN film forming temperature in the first step, and a film of an AlxInyGa1-x-yN layer (0<x≤1, 0≤y≤1, x+y≤1) is formed at a temperature lower than the GaN film forming temperature, and a film of GaN is formed at the GaN film forming temperature in the second step.
The below Documents 2 to 4 disclose methods for doping a nitride semiconductor layer with C (carbon) or the like, to improve tolerance of a semiconductor device including a nitride semiconductor layer. More specifically, the below Document 2 discloses a composite semiconductor substrate in which a multilayer buffer layer and nitride active layer are deposited in series on a Si single-crystal substrate, wherein AlxGa1-xN single-crystal layers (0.6≤x≤1.0) including C at 1*1018 to 1*1021 atoms/cm3 and AlyGa1-yN single-crystal layers (0.1≤y≤0.5) including C at 1*1017 to 1*1021 atoms/cm3 are alternately and repeatedly deposited in this order in the multilayer buffer layer, and the nitride active layer consists of an electron transition layer in which the C containing density is equal to or less than 5*1017 atoms/cm3 and an electron supply layer. The C containing densities in the AlxGa1-xN single-crystal layer and the AlyGa1-yN single-crystal layer decrease from the substrate side toward the active layer side.
The below Document 3 discloses a nitride semiconductor wafer including a substrate and a buffer layer on the substrate, wherein the buffer layer includes an alternating layer of AlxGa1-xN layer (0≤x≤0.05) and AlyGa1-yN layer (0<y≤1, and x<y). In the alternating layer, only the AlyGa1-yN layer includes acceptors.
Further, the below Document 4 discloses a nitride semiconductor element which includes a base substrate, a buffer layer formed above the base substrate, an active layer formed on the buffer layer, and at least 2 electrodes formed above the active layer. The buffer layer has one or more composite layers which include a plurality of nitride semiconductor layers having different lattice constants. In at least one of the composite layers, a carrier area of the nitride semiconductor layer having the largest lattice constant among the plurality of nitride semiconductor layers is intentionally doped with carbon atoms at a density beforehand determined and oxygen atoms at a density beforehand determined.